There are many needs for sensors that optically couple a laser output to a target and also couple the reflected light from the target to a measuring device. The principal of operation of such devices is that light from a laser is coupled through an optical fiber and focused on a target. Light reflecting from (or emitted by) the target is coupled through an optical fiber to the measuring device. Subject to limitations of cost, size, and efficiency, such devices can be used for applications such as compact disk read or read/write devices, for spectral analysis of samples, and for measuring shock waves.
One approach to this problem is to use a single optical fiber to couple the light both to the target and, after reflection, back to a sensor. Such as system is shown in U.S. Pat. No. 4,154,529 of Dyott. There are several disadvantages to such a system. Since the light is sent and received on the same fiber, there is a major problem of signal mixing, that is, some of the outgoing light is detected as a return signal before it gets to the target. One major contributor to this phenomenon is Fresnel reflections which occur at every juncture where the light leaves the fiber and goes into air or some other medium with a different index of refraction. Typical Fresnel reflections are about 4% of the incident light per surface, so a connector junction where two fibers are butted together would have nearly 8% of the initial laser power mixed with the light returning from the target. Add that signal to other Fresnel reflections in the system and it is immediately apparent that this phenomenon can obscure any true return light from a system. There are many instances where the return light is less than 1% of the input light signal which would be swamped by the Fresnel reflections. In addition, the system for sending and collecting is much more complex and expensive than a dual fiber system. Since the input and return light are emanating from the same place, i.e. the end of the fiber that is coupled to the laser, separation equipment, and expensive optical components must be used and precisely aligned and maintained.
Another approach is to use two parallel optical fibers that are aligned perpendicular to a target, as shown in U.S. Pat. No. 4,739,161 of Moriyama et al. This patent does not suggest the use of a lens to concentrate the light, but keeps the fiber ends close enough to the target that sufficient reflected energy is received. Since the simple form of this invention (shown in FIGS. 10 and 11 of the '161 patent) does not have high sensitivity, the preferred embodiments use multiple optics and complicated signal processing to enhance sensitivity
U.S. Pat. No. 4,801,799 of Tromborg et al. shows at FIG. 3 an alternative approach using two parallel optical fibers located at the focal point of a lens, so collimated light from a first fiber is transmitted from the lens to a reflective area on a vibrating surface, and reflected collimated light is received by a different portion of the lens and transmitted to the second fiber. The principle of operation of Tromborg's system is believed to be different from that stated by that patent, because movement of the vibrating stage towards and away from the lens would not change the focal position to `b` and `c` as shown in FIG. 3 of that patent, as the collimated light emanating from the reflective surface and passing through the lens will always return to the focal point `a` of the lens. One explanation for the operation of Tromborg's system is that the vibrating mirror 34 is at an angle other than perpendicular to the path of light from fiber 14, so that the reflection is directed towards output fiber 16. In this event, the mirror or the optics assembly must be on a precise tilt stage in order to reflect the light back into the return fiber, and the reflective surface can only be a high quality mirror. This is a serious limitation in that a very narrow range of reflectors can be used and if that reflector is slightly misaligned either during operation or during setup, it will not work.
U.S. Pat. No. 5,202,558 of Barker shows as sensor that has a first optical fiber coupled from a laser to a graded index (GRIN) lens mounted axially in a housing. Light from the GRIN lens reflects off a target and is collected by a lens system axially mounted in the housing behind the GRIN lens. The lens system focuses the received light on an output optical fiber at the rear of the housing. The efficiency of this device is compromised by the shadow cast by the GRIN lens on the reflected light. The device is also incapable of being reduced in size as the housing must be large enough for most reflected light to pass around the GRIN lens. Furthermore, the GRIN lens has a short focal length compared to the collection lens. This short focal length is mandatory since the lens must capture the diverging light from the fiber, then focus it onto the target. Image magnification (in this case the diameter of the fiber is the image size) can be simply stated as the ratio of the focal lengths of the sending lens (GRIN) and the collecting lens. The Barker design will expand the image (spot diameter on the target) by approximately 6, which means that if a pair of 200 um diameter fibers were used, the spot on the target would be six times larger, or 1200 um diameter. The collecting lens takes that image diameter and tries to focus it into the 200 um receiving fiber and severely overfills the fiber by a factor of six. Using the area ratio of the diameter of the fiber versus the overfill diameter of 1200 um, the light collection is 36 times less efficient than the instant invention.